The uplinks are allways monitored by the connection monitor. Both uplinks are monitored continously. If the primary uplink fails, the MX switches to the secondary uplink. In case of loadbalancing, the failed uplink is not used any more. Failback will be delayed by 15 seconds to deal with flapping links.
Maybe your specific conditions let the MX not recognize the bad link.
You can buy the SD-WAN Plus license and configure performance classes to make the uplink decisions (Security & SD-WAN > Configure > SD-WAN & Traffic Shaping).
Notice: Without SD-WAN Plus license these performance classes can only be used with Auto VPN traffic, not with internet traffic.